In a hospital, it is sometimes necessary to monitor a patient in order to ensure that the patient remains in bed. For example, a patient who is physically capable of leaving the bed but who is subject to dizziness upon standing up might fall and injure himself if he leaves the bed when no attendant is present. Consequently, it is desirable that an alarm be given in the event the patient leaves the bed so that a member of the hospital staff will be alerted to the fact that the patient has left the bed and will ensure that the patient does not experience an injury.
An early technique for automatically detecting patient exit involved the provision of a normally-closed mechanical switch in the mattress or springs of a bed, the weight of the patient being sufficient to open the switch, and the absence of the patient permitting the switch to assume its closed condition. The switch could be connected in a series circuit with a source of power such as a battery and with an alarm such as a buzzer, and would thus produce audible noise when the patient left the bed.
Modern hospital beds typically have a supporting frame which supports the mattress and the patient, the supporting frame being itself supported on the bed by a plurality of load cells which each produce an electrical signal representative of the respective mass or weight supported by that load cell. In a conventional system of this type, the partial weight values from the various load cells can be periodically summed in order to produce a total weight value, and this value is monitored for a reduction which exceeds a predetermined limit value. If the total measured weight drops to a value below the limit value, it is interpreted to mean that the patient has substantially or completely exited the bed. One known system of this type is disclosed in Koerber, Sr. et al U.S. Pat. No. 4,934,468. While known systems of this type have been adequate for their intended purposes, they have not been satisfactory in all respects.
For example, a substantial portion of the weight of the patient must be removed from the patient supporting frame before the system is capable of determining that the patient is exiting the bed. In the case of a patient who is attempting to stand up, this means that the patient must be supporting a significant portion of his own weight on the floor before the system can detect that the patient is attempting to exit the bed. Thus, the patient will be substantially on his own feet and may be starting to experience a dizzy spell before any alarm is given, with the possibility that the patient may fall and injure himself before hospital personnel can reach him. It would be desirable to be able to detect the exit sooner, for example where the patient has moved himself to a sitting position on the edge of the bed but has not yet placed his feet on the floor and thus has all of his weight still supported by the patient supporting frame.
It is therefore an object of the present invention to provide a patient exit detection apparatus which is capable of accurately detecting patient exit while all or at least a substantial portion of the patient's weight is still supported by a patient supporting frame of the bed.
A further object is to provide such a system which does not require significantly more structure than existing systems and is thus cost-competitive with existing systems, and which in the case of a microprocessor-based bed can be implemented solely through a software change and can thus be easily retrofit into preexisting microprocessor-based beds.